auto merge of #7443 : yjh0502/rust/fix_field_dup, r=huonw

Check if there is duplicated field names in struct.

src/test/compile-fail/struct-fields-decl-dupe.rs

@@ -0,0 +1,17 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct BuildData {
+    foo: int,
+    foo: int, //~ ERROR field `foo` is already declared
+}
+
+fn main() {
+}